Integrated optics polarization beam splitter using form birefringence

ABSTRACT

A method for separating the orthogonal polarization components of an incident optical signal into two spatially separated output ports is described. The method comprises a Mach-Zehnder interferometer where one of the two branches has a section of waveguide that exhibits form-birefringence. This integrated optic Polarization Beam Splitter (PBS) is broadband, has high extinction ratio, and has characteristics that are tunable.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation in part of the patentapplication identified by U.S. Ser. No.10/661,891 filed on Sep. 15,2003, the entire content of which is hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a method of spatially separating the twoorthogonal polarization states of an incident optical signal. Itsprimary use is in integrated optics, where it is often desirable tosplit and manipulate an optical signal's orthogonal polarizationsindependently (polarization diversity). It can also be used inpolarization mode dispersion (PMD) compensating devices, where the twoorthogonal polarizations must be split, processed, then recombined.

(2) Brief Description of Related Art

Light is a vector field that has two primary and orthogonal polarizationstates or vector directions. These are sometimes referred to as the Sand P polarizations in free space optics, or the TE (TransverseElectric) and TM (Transverse Magnetic) modes of optical waveguides. Theperformance of optical waveguides and optical devices is often sensitiveto the polarization state. That is, the response of the device changesas the polarization state changes. This is particularly pronounced inintegrated optical waveguides that are fabricated on dielectricsubstrates.

Typically, it is desirable to have optical components that areinsensitive to the input state of polarization. This criteria arisesfrom the fact that in fiber optic telecommunications, the polarizationstate of an optical signal that has traveled down any length of fiber isunknown, random, and time varying (due to perturbations in theenvironment). Great care is often taken in the design and fabrication ofoptical components so that they behave in a polarization insensitivemanner. Despite this effort, most devices remain polarization sensitiveto some degree, and this affects ultimate performance, yield, and cost.On the other hand, there are some special applications where the twopolarization states of an input optical signal needs to be spatiallysplit so each can be manipulated independently. This is the case forexample, in PMD (Polarization Mode Dispersion) compensators, where thedispersion of the signal on the two states needs to be equalized. Inapplications where the polarizations need to be split, the extinctionratio, which is the ratio of wanted to unwanted polarization in eitherof the two branches, must be high.

Another general way to handle polarization in a device that is requiredto behave as if it were polarization insensitive is to split the inputpolarization into two branches having orthogonal states, process eachbranch independently with devices that are optimized for eachpolarization respectively, and then recombine the processed signalstogether. This scheme is referred to as “polarization diversity”. It hasthe advantage that each branch can be specifically optimized for itsrespective polarization, giving the best performance without otherwisehaving to comprise the ability to give adequate performance over twopolarization states simultaneously. The drawbacks are that twice thenumber of devices are required, and two polarization splitters areneeded to split then recombine the signals. Naturally this adds cost andcomplexity, but the objective is to net an overall superior performingor higher yielding component.

Traditionally, optical components have been quite large, andpolarization diversity schemes have not been popular because of theadded size and cost associated with packaging twice the componentry plusthe splitters. Prospects for polarization diversity improve forintegrated optics fabricated on substrates, where the objective is toshrink the size of components and to integrate various functionalitieson a common die or chip, similar in concept to integrated electroniccircuits (ICs). In this case the polarization splitters and two sets ofcomponents are fabricated all at once. Future integrated opticalcomponents are miniaturized by the use of high-index contrastwaveguides. High-index waveguides themselves are more susceptible topolarization sensitivity. Polarization diversity may be the only pathforward for these future high-index contrast components.

Prior Art:

Most polarization beam splitters are bulk optic, and make use ofbirefringent wave plates. We will not discuss bulk optic polarizationsplitters here, but only emphasize integrated optic versions.

U.S. Pat. No. 5,946,434 discusses an integrated optic Y-couplerpolarization splitter. The splitter works by taking advantage of thedifference in waveguide-to-waveguide coupling strengths for twoorthogonal polarizations. The optimum structure is a result of anoptimized coupling length. Both the coupling length, and the propagationconstants are wavelength dependent, and therefore the polarizationsplitter will have a wavelength dependence, which is undesirable.

U.S. Pat. No. 5,475,771 discusses an integrated optic Y-branchingwaveguide where one of the branches contains an anisotropic material.The structure requires the integration of an anisotropic material on tothe integrated substrate. Such integration is not desirable because thetwo materials are not well matched in index (leading to scatteringloss). Also the fabrication introduces additional steps that impactperformance, cost, and yield. Most anisotropic materials can not bedeposited by methods used to form the dielectric waveguides.

U.S. Pat. No. 5,293,436 discusses an integrated optic Mach-Zehnderinterferometer wherein one branch contains a potable material. Polablematerials do not have long term stability, and are not used widely intelecom grade components. The poled materials tend to relax with acertain time constant (that is also affected by environmentalconditions), and the performance degrades over time. Further, onlycertain materials are potable, and very few such materials make goodpassive low loss optical waveguides.

U.S. Pat. No. 5,151,957 discusses an integrated optic delta-beta couplerconfiguration in X-cut Lithium Niobate. This method only works inLithium Niobate, and is therefore not compatible with general integrateoptic waveguides and materials.

U.S. Pat. No. 5,133,029 discusses an integrate optic 2×2 beam splitterwherein the set of Y-junctions comprise waveguides of different widths.The waveguides forming the Y-junctions of this device must be comprisedof anisotropic materials, and therefore limits the scope of thisinvention to those integrated optic waveguides using such materials(which is few).

U.S. Pat. No. 5,111,517 discusses an integrated optic Mach-Zehnder inX-cut Lithium Niobate. This method only works in Lithium Niobate, and istherefore not compatible with general integrate optic waveguides andmaterials.

U.S. Pat. No. 5,056,883 discusses an integrated optic Y-branchingwaveguide where in one branch contains a glassy polable polymer. Thisinvention is similar to U.S. Pat. No. 5,475,771 above, where theanisotropic material is specifically an anisotropic polymer material (ora polable polymer material) that is deposited over only one branch ofthe Y-branching waveguide.

U.S. Pat. No. 4,772,084 discusses an integrated optic 3×3 coupler. Thisinvention is similar in its physical mechanism for polarizationsplitting as that described in U.S. Pat. No. 5,946,434 above, exceptthat it uses a three-waveguide coupler instead of a two-waveguidecoupler, and provides electrodes for post fabrication thermal orelectro-optic trimming.

SUMMARY OF THE INVENTION

FIG. 1 shows a schematic of an asymmetric four-port Mach-Zehnderinterferometer comprised of two 3-dB couplers 11, 14 or power splittersand two branches 12,13 of lengths L₁ and L₂ as indicated. Consider thecase where the structure is lossless, the waveguides in each branch areidentical, and the TE and TM propagation constants of the waveguides areidentical. Then the responses at the two output ports are sinusoidal asa function of the branch length difference L₁-L₂, and are identical forboth the TE and TM polarization states.

More generally, consider the asymmetric four-port Mach-Zehnder in FIG. 1wherein the waveguides in paths 1 and 2 are not identical, and therespective propagation constants of each of the waveguides arepolarization sensitive. Then the responses are sinusoidal functions ofthe optical phase difference between the two paths for eachpolarization, given as $\begin{matrix}{{\Delta\quad\phi^{e}} = {\frac{2\pi}{\lambda}( {{N_{1}^{e}L_{1}} - {N_{2}^{e}L_{2}}} )}} & (1) \\{{\Delta\quad\phi^{h}} = {\frac{2\pi}{\lambda}( {{N_{1}^{h}L_{1}} - {N_{2}^{h}L_{2}}} )}} & (2)\end{matrix}$where Δφ^(e) and Δφ^(h) are the phase differences for the TE and TMmodes respectively, N^(e) ₁ and N^(e) ₂ are the modal effective indexesof the TE mode in branch 1 and branch 2 respectively, N^(h) ₁ and N^(h)₂ are the modal effective indexes of the TM mode in branch 1 and branch2 respectively, and λ is the wavelength. In the lossless case, outputports 1 and 2 are complementary. That is, the sum of the power at thetwo output ports is equal to the input power.

The objective of a polarization splitter in this invention is to haveone polarization appear at output port 1, and the orthogonalpolarization to appear at output port 2. It is also an objective tominimize the unwanted polarizations at each port. A figure of meritcommonly used is the Extinction Ratio (E.R.). This is the ratio ofwanted to unwanted power in each polarization for each port. In theMach-Zehnder configuration, one output port for one polarization willhave maximum transmission when the phase difference between paths isequal toΔφ^(e)=π+2Nπ, where N is some integer  (3)The other output port, for the second polarization will have a maximumwhen the phase difference is equal toΔφ^(h)=2Mπ, where M is some integer  (4)

When the transmission is a maximum in one output port, it will be aminimum in the other output port. The design criteria for constructing apolarization splitter is to chose the path lengths L₁ and L₂, and theeffective indexes N^(h) ₁, N^(e) ₁, N^(h) ₂, and N^(e) ₂ in such a waythat equations (3) and (4) are simultaneously satisfied for some set ofintegers N and M.

In any polarization splitter design based on a Mach-Zehnderconfiguration, one must be able to design and fabricate waveguides thathave substantially different propagation constants for the TE and TMmodes. The term “birefringent” is used to describe the condition wherethe TE and TM modes of a single waveguide have different propagationconstants. “Small” and “large” birefringence are terms used to describeconditions where the TE and TM modes are nearly identical, and far fromidentical, respectively. In the literature and in patent disclosures,birefringence is typically induced by poling a material having certainsymmetries, such as Lithium Niobate, or by the anisotropic electro-opticeffect in certain materials such as Lithium Niobate or Indium Phosphide.These types of birefringences are termed material birefringence becausethe material exhibits different indexes of refraction depending on thepolarization state.

The invention described here makes use of form birefringence, also knownas waveguide birefringence, and does not rely on material birefringence.Form birefringence is related to the waveguide geometry and structure,and can be induced in a number of ways, including the following.

-   1. Changing the width of a waveguide. (FIG. 2) Changing the width of    a waveguide with a lower cladding 21, a core 221, an upper cladding    23, and a cover 24 (while its thickness remains the same) changes    both the average effective index, and the difference between the TE    and TM effective indexes. In low index contrasts waveguides (where    the core-to-cladding refractive index difference is less than about    0.02), the birefringence induced by changing the waveguide width is    very small. However, as the index contrast increases, so does the    change in birefringence. For index contrasts larger than 0.05, the    induced birefringence is large enough to realize robust polarization    splitters, as we demonstrate. FIG. 2 shows a non-birefringent square    waveguide 221, and a waveguide 222 having birefringence induced by    narrowing the width.-   2. Creating non-homogeneous waveguides. A homogeneous waveguide is    one where the refractive index of the waveguide core is the same    everywhere within the guiding core, and the refractive index of the    cladding is the same everywhere within the vicinity of the core    (practically, within 10 um of the core). Non-homogeneous means that    the index within the core, or within the cladding, has a spatial    variation. Layered or striated materials are also considered    non-homogeneous. A non-birefringent waveguide can be made    birefringent by placing a thin high-index layer 223 (higher index    than the core) above or beneath the guide, as shown in FIG. 3.-   3. Birefringent Material Overlay. The method of 2 above can be    generalized to a thin layer 224 of any index, but having a material    birefringence. Example of overlays are stressy SiN, or polymers, as    shown in FIG. 4.

Form birefringence is a method to design the effective indexes N^(h) ₁,N^(e) ₁, N^(h) ₂, and N^(e) ₂ independently. This design freedom, inaddition to the ability to specify L₁ and L₂, means that equations (3)and (4) can be satisfied simultaneously, and therefore, polarizationsplitters can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an asymmetric four-port Mach-Zehnderinterferometer.

FIG. 2 shows the cross-section of a birefringent waveguide with narrowercore.

FIG. 3 shows the cross-section of a birefringent waveguide with a thinhigh index layer.

FIG. 4 shows the cross-section of a birefringent waveguide with a thinbirefringent layer.

FIG. 5 shows the design of a form-birefringent waveguide.

FIG. 6 shows the top-down view of the first embodiment of the waveguidesfor the polarization splitter of the present invention.

FIG. 7 shows the top-down view of the second embodiment of thewaveguides for the polarization splitter with a heated section.

FIG. 8 shows the top-down view of the third embodiment of the waveguidesfor the polarization splitter using a Mach-Zehnder balanced coupler as3-dB couplers.

FIG. 9 shows data taken from a fabricated polarization-splitter shown inFIG. 8.

FIG. 10 shows the top-down view of a fourth embodiment of the waveguidesfor the polarization splitter.

FIG. 11 shows the polarization splitter as a functional block diagramand shows two operating states of the device.

FIG. 12 shows a cascade of polarization splitters used to increase theextinction ratio and optimize the performance.

FIG. 13 shows a cascade of polarization splitters used to increase andequalize the extinction ratio and optimize the performance.

DETAILED DESCRIPTION OF THE INVENTION

Consider the waveguide structure that is shown in cross section in FIG.5. The core material is silicon oxynitride (SiON) with a refractiveindex of n_(co)=1.70. In one embodiment, the upper and lower claddingsare silica (SiO₂) and thermal oxide (thermally grown SiO₂) respectively,both with an index of n_(cl)=1.45. SiON and SiO₂ can be deposited bychemical vapor deposition (CVD), which is well known in the integratedoptics and semiconductor fields. Silica can also be deposited by flamehydrolysis (FHD), or by sputtering. Other suitable core materialsinclude silicon nitride (SiN), silicon (Si), silicon oxycarbide andTantalum oxide-silica (Ta₂O₅:SiO₂). The Tantalum oxide-silica istypically sputter deposited, and the other suitable core materials aretypically deposited by CVD (chemical vapor deposition), or LPCVD (Lowpressure CVD), and can also be sputtered. Other materials suitable forthe core and/or the upper and/or lower cladding include Indium Phosphidecompounds, Gallium Arsenide compounds, and high index contrast polymers.As will be understood by one skilled in the art, the constituent partsof the materials forming the core, upper cladding and/or lower claddingcan be varied in accordance with the present invention to provide apreferable core to cladding index contrast (n_(co)-n_(cl)) larger than0.05, and we call such contrasts “high-index contrast”. The desiredwaveguide height h can vary between 0.5 um to 4.0 um for high indexcontrast guides operating at a wavelength near λ=1.55 μm. Here it isselected as h=1.5 μm. The height is typically chosen so that thewaveguide is single moded at the wavelength of interest. The width wwill be varied to give a certain amount of form birefringence.

Using rigorous numerical models (Apollo Photonics, Inc. OWMS Suite), itis found that the effective indexes for the TE mode (N^(e)) and the TMmode (N^(h)) at a wavelength of 1.55 μm follow the relations below as afunction of waveguide width w,N ^(e)=1.64233−0.325 exp [−1.5456w]  (5)N ^(h)=1.63563−0.325 exp [−1.5456w]+0.0547 exp [−1.339w]  (6)The birefringence, which is the difference between the TE and TMeffective indexes is,N ^(e) −N ^(h)=0.0067−0.0547 exp [−1.339w]  (7)For a waveguide width of w=1.50 μm (square waveguide), the birefringenceapproaches zero.

The waveguide structure described above is used in the Mach-Zehnderconfiguration depicted in FIG. 6. The Mach-Zehnder consists of twodirectional coupler type 3-dB couplers 111, 112 and 141, 142. Thenominal waveguide width in the couplers is 1.50 μm. The coupler lengthsare chosen from simulation to be 45 μm, and the cores are separated by0.7 μm. All the waveguide bends have radii of 300 μm. The nominal widthof the waveguides is 1.5 μm. The path lengths in the Mach-Zehnderbranches are set equal here, L₁=L₂=L_(mz). There is a section ofwaveguide 122 (labeled w₂) having a width of 0.8 μm in one arm of theinterferometer. In this section the waveguide is tapered from a width of1.5 μm to 0.8 μm over a length of 8 μm. By narrowing the waveguide to awidth of 0.8 μm, a certain amount of birefringence is induced accordingto equation (7). The length of the narrowed section is chosen to be thepolarization beat length, L_(p). The polarization beat length is thelength over which the TE and TM modes accumulate a phase difference ofπ. It is given by the relation, $\begin{matrix}{L_{p} = {\frac{\lambda}{2( {N^{e} - N^{h}} )}}} & (8)\end{matrix}$From (7) L_(p) is calculated to be 64 μm at λ=1.55 μm. As outlinedearlier in equations (3) and (4), two conditions must be met to have ahigh extinction ratio. Relation (8) is one condition. In order tosatisfy (3) and (4) simultaneously, one can vary the waveguide width W₂simultaneously with varying L_(p). Another method is to change thedifference in path lengths between the upper and lower branches of theMach-Zehnder (L₁ and L₂ from FIG. 1). A third method is to change theindex of one of the branches by use of the thermal optic effect. FIG. 7shows the forgoing polarization splitter with a resistive heater 132placed over one of the arms. Current injected into the resistor willheat that arm and can be used as a tuning or trimming mechanism. Theheater changes the effective indexes of both polarizations by nearly thesame amount, and does not itself induce significant birefringence. Theheater used in the demonstration consisted of 200 nm of platinumdeposited by an evaporator.

A further improvement is shown in FIG. 8, where the simple directionalcoupler type 3-dB couplers depicted in FIGS. 6 an 7 are replaced byMach-Zehnder balanced coupler¹ 113,114 and 143,144. The balancedcouplers are 3-dB couplers with improved fabrication latitude and aremore wavelength-independent compared to conventional directionalcouplers.¹ B. E. Little et. al. “Design rules for maximally-flatwavelength-insensitive optical power dividers using Mach-Zehnderstructures”, Optics Lett. Vol., pp. 1998.

Data taken from the fabricated device in FIG. 8 is shown in FIG. 9. Thenumeric labels correspond to the port labeling of FIG. 8. The graph is aplot of extinction ratio as a function of thermal tuning power appliedto the resistive heater. Extinction ratio is the ratio of the power inone polarization state (wanted polarization) compared to the other state(unwanted polarization). As seen, the extinction ratios can be tuned upto 25 dB. Thus the heater gives a post fabrication method to optimizethe performance. These polarization splitters can be cascadedoutput-to-input to increase the extinction ratios.

FIG. 10 shows the top view of another embodiment of the invention. Thestructure is similar to that shown in FIG. 6. Compared to FIG. 6, inthis case there is no narrow section of waveguide on the upper branch.Instead, there is a section 123 of waveguide on the upper branch thathas a thin layer of additional material. The material can be a thin highindex layer as described in conjunction with FIG. 3, or a thinbirefringent layer as described in conjunction with FIG. 4. The lengthof waveguide having this material layer is L_(p). This thin layer ofhigh index or birefringent material can replace the narrow section ofwaveguide of width w₂ in FIGS. 7 and 8.

FIG. 11 shows a functional block diagram of the polarization splitters150 and 151. The device has two inputs labeled “1” and “3”, and twooutputs labeled “2” and “4”, similar to the physical structure shown inFIG. 8. The polarization splitter separates the constituent polarizationstates P1 and P2 at the input into two physically separated outputports. In an optimized configuration, substantially all of the power inone polarization state comes out one output port, while substantiallyall of the power in the other polarization state comes out the secondoutput port. There are two possible optimum states for this device asshown in FIG. 11. In one state, labeled as “state 1” in FIG. 11 150,substantially all of the power in polarization P1 comes out of outputport 2, while substantially all of the power in polarization P2 comesout of output port 4. In the second state, labeled as “state 2” in FIG.11 151, substantially all of the power in polarization P1 comes out ofoutput port 4, while substantially all of the power in polarization P2comes out of output port 2. It is possible switch from state 1 to state2 by applying a bias 152,153 to a thermal heater on the structure asindicated previously in FIGS. 7, 8, 9. FIG. 9 shows experimentalevidence that the extinction ratio can be controlled and changed byapplying energy to the thermo-optic heater. We say that the polarizationstate at the output of the device can be “toggled” between two states byapplying a bias in a manner consistent with FIGS. 7-9.

FIG. 12 shows a cascade of polarization splitters 160 used to increasethe extinction ratio at the final output ports. In this case there arethree polarization splitters labeled “A” 161, “B” 162, and “C” 163. Theinput is first connected to polarization splitter A. Polarizationsplitter A has some bias “bias 1” 164 so as to operate in some statecalled “state 1” similar to that shown in FIG. 11, where polarization P1comes out of port 2 of splitter A, and polarization P2 comes out of port4 of splitter A. Port 2 of splitter A becomes the input to port 1 ofsplitter B. Port 4 of splitter A becomes the input to port 1 of splitterC. Splitters B and C are similar to splitter A. FIG. 12 connectsidentical devices in an output-to-input configuration. All devices areoperated in the same bias mode or state, with perhaps some trimming onthe biases to compensate for fabrication deviations. The finalextinction ratios of the output ports are a multiplication of theextinction ratios of each port of each device.

The optimized extinction ratios for the polarizations at output ports 2and 4 with input at port 1 (see FIG. 11), may not be identical, as shownby experimental values in FIG. 9. For example, FIG. 9 shows that theextinction ratio for the polarization that goes from port 1 to port 4 isbetter than for the polarization that goes from port 1 to port 2.Therefore, the extinction ratios for polarization P1 and for P2 in thecascade configuration shown in FIG. 12 may not be identical. One methodto make them identical is to operate the cascade such that splitter A,and splitters B and C are operated in different states, as shown in FIG.13. Note that the bias on splitter A 164 is different than that bias onsplitter B and C 165. For example, by tracing out polarization state P1,the extinction ratio for P1 is a product of going from ports 1 to 2 insplitter A, and from ports 1 to 4 in splitter B. Likewise tracing outpolarization state P2, the extinction ratio for P2 is a product of goingfrom ports 1 to 4 for in splitter A, followed by going from ports 1 to 2in splitter C. Therefore the extinction ratios at the final two outputports are equalized.

While the preferred embodiments have been described, it will be apparentto those skilled in the art that various modifications may be made tothe embodiments without departing from the spirit of the presentinvention. Such modifications are all within the scope of thisinvention.

1. An optics polarization beam splitter for separating orthogonalcomponents of an incident optical signal using an asymmetricMach-Zehnder interferometer, comprising: an input optical coupler tosplit the incident optical signal, which has two orthogonal polarizationstates, into a first waveguide branch and a second waveguide branchwherein said first waveguide branch has a birefringent section thatexhibits form birefringence birefringence to separate said twoorthogonal polarization states, the birefringent section of the firstwaveguide branch constructed of core material encapsulated by an uppercladding and a lower cladding with the birefringent section having acore to cladding index contrast being a high index contrast; and anoutput optical coupler to combine the optical signals outputted fromsaid first waveguide branch and said second waveguide branch and tooutput two orthogonal output optical signals.
 2. The optics polarizationbeam splitter as described in claim 1, wherein said optical inputcouplers and said optical output coupler are 3-dB couplers.
 3. Theoptics polarization beam splitter as described in claim 1, wherein saidtwo orthogonal polarization states are transverse electric (TE) mode andtransverse magnetic (TM) mode.
 4. The optics polarization beam splitteras described in claim 1, wherein said birefringent section of said firstwaveguide branch has a width narrower than a width of said secondwaveguide branch.
 5. The optics polarization beam splitter as describedin claim 1, wherein said birefringent section of said first waveguidebranch has a width wider than a width of said second waveguide branch.6. The optics polarization beam splitter as described in claim 4,further comprising a heater in proximity to said second waveguidebranch.
 7. The optics polarization beam splitter as described in claim4, further comprising a heater in proximity to said birefringent sectionof said first waveguide branch.
 8. The optics polarization beam splitteras described in claim 1, wherein said input optical coupler and saidoutput optical coupler are Mach-Zehnder balanced couplers.
 9. The opticspolarization beam splitter as described in claim 1, wherein the corematerial of said birefringent section is coated with a high index ofrefraction layer.
 10. The optics polarization beam splitter as describedin claim 1, wherein the core material of said birefringent section iscoated with a birefringent layer.
 11. The optics polarization beamsplitter as described in claim 1, wherein said core material is selectedfrom the group consisting of silicon oxynitride (SiON), silicon nitride,silicon oxycarbide, silicon (Si), and tantalum oxide-silica(Ta₂O₅:SiO₂).
 12. The optics polarization beam splitter as described inclaim 1, wherein the upper cladding is constructed of silica (SiO₂); andthe lower cladding is constructed of thermal oxide.
 13. The opticspolarization beam splitter as described in claim 1, further comprising acascade of beam splitters connected between said input optical couplerand said output optical coupler.
 14. The optics polarization beamsplitter as described in claim 6, wherein the heater is used to togglethe states of polarization at the output of said output optical coupler.15. The optics polarization beam splitter as described in claim 7,wherein the heater is used to toggle the states of polarization at theoutput of said output optical coupler.
 16. The optics polarization beamsplitter as described in claim 13, wherein a first stage of said cascadeis operated in one polarization state, and a second stage of saidcascade is operated in a second polarization state.
 17. The opticspolarization beam splitter as described in claim 1, wherein the core tocladding index contrast in the birefringent section is larger than 0.05.18. The optics polarization beam splitter as described in claim 1,wherein the first waveguide branch and the second waveguide branch areencapsulated by the lower cladding and the upper cladding, and whereinthe first and second waveguide branches have a core to cladding indexcontrast being a high index contrast.
 19. The optics polarization beamsplitter as described in claim 18, wherein the core to cladding indexcontrast between the lower cladding, upper cladding and first waveguidebranch is larger than 0.05, and wherein the core to cladding indexcontrast between the lower cladding, upper cladding and second waveguidebranch is larger than 0.05.
 20. An optics polarization beam splitter forseparating orthogonal components of an incident optical signals using anasymmetric Mach-Zehnder interferometer, comprising: an input opticalcoupler to split an incident optical signal, which has two orthogonalpolarization states, into a first waveguide branch and second waveguidebranch wherein said waveguide branch has a birefringent section thatexhibits form birefringence to segregate said two orthogonalpolarization states, and has a core material coated with a birefringentlayer to cause the form birefringence in the birefringent section; andan output optical coupler to combine the optical signals outputted fromsaid first waveguide branch and said second waveguide branch and tooutput two orthogonal polarization output optical signals.
 21. Theoptics polarization beam splitter as described in claim 20, wherein saidoptical input couplers and said optical output coupler are 3-dBcouplers.
 22. The optics polarization beam splitter as described inclaim 20, wherein said two orthogonal polarization states are transverseelectric (TE) mode and transverse magnetic (TM) mode.
 23. The opticspolarization beam splitter as described in claim 20, wherein saidbirefringent section of said first waveguide branch has a width narrowerthan a width of said second waveguide branch.
 24. The opticspolarization beam splitter as described in claim 20, wherein saidbirefringent section of said first waveguide branch has a width widerthan a width of said second waveguide branch.
 25. The opticspolarization beam splitter as described in claim 23, further comprisinga heater in proximity to said second waveguide branch.
 26. The opticspolarization beam splitter as described in claim 23, further comprisinga heater in proximity to said birefringent section of said firstwaveguide branch.
 27. The optics polarization beam splitter as describedin claim 20, wherein said input optical coupler and said output opticalcoupler are Mach-Zehnder balanced couplers.
 28. The optics polarizationbeam splitter as described in claim 20, wherein said core material isselected from the group consisting of silicon oxynitride (SiON), siliconnitride, silicon oxycarbide, silicon (Si), and tantalum oxide-silica(Ta₂O₅:SiO₂).
 29. The optics polarization beam splitter as described inclaim 20, further comprising an upper cladding and a lower claddingcooperating to encapsulate the first waveguide branch, and wherein theupper cladding is constructed of silica (SiO₂); and the lower claddingis constructed of thermal oxide.
 30. The optics polarization beamsplitter as described in claim 20, further comprising a cascade of beamsplitters connected between said input optical coupler and said outputoptical coupler.
 31. The optics polarization beam splitter as describedin claim 25, wherein the heater is used to toggle the states ofpolarization at the output of said output optical coupler.
 32. Theoptics polarization beam splitter as described in claim 26, wherein theheater is used to toggle the states of polarization at the output ofsaid output optical coupler.
 33. The optics polarization beam splitteras described in claim 30, wherein a first stage of said cascade isoperated in one polarization state, and a second stage of said cascadeis operated in a second polarization state.
 34. The optics polarizationbeam splitter as described in claim 20, wherein the core to claddingindex contrast in the birefringent section is larger than 0.05.